In prior lessons, we have seen that genes are blueprints for proteins. Specifically, a gene contains the information needed to build a protein out of amino acids. Furthermore, we’ve seen that differences in the properties of the various amino acids make it possible for them to assemble into an incredible diversity of proteins. Now, we’ll take a more in-depth look at proteins and how they work.
From Genes to Proteins
While it is true that genes are responsible for all our physical characteristics, such as eye color, height, or predisposition to certain diseases, genes do not act directly to give these characteristics physical form. Instead, all these attributes are the result of the activities of proteins, which are built according to the genetic code discussed in Lesson 4. If you recall, the genetic code uses an alphabet of only four nucleotides (typically represented as A, C, G, and T) to code for the construction of all the many proteins used by every form of life on the planet! This genetic code is translated by the machinery of the cell into a single, long chain of amino acids called a polypeptide. As we discussed in our previous lesson, when this polypeptide is complete, the chemical properties of its amino acids cause it to fold into its final three-dimensional structure. This folding of the protein into its final shape is a small but important step.
The Importance of 3D Structure
The final shape that a protein assumes determines its function; for instance, the large class of proteins called enzymes are usually globular, with one or more “pockets” on the surface of the protein that bind another molecule. The shape of these pockets is so specific that it will usually only accept one type of molecule, which means that a different type of enzyme is needed for each molecule that a cell must make or break down. For instance, the protein lactase has a pocket that only recognizes the sugar lactose, which is found in milk. Having bound this molecule, it can then break the chemical bonds that hold the sugar molecule together, releasing energy for the cell to use. Other proteins have pockets that allow them to bring small molecules together to form larger molecules.
However, proteins aren’t just limited to breaking down or building other molecules. Structural proteins may be embedded in cell membranes or assembled into long chains to provide support for different parts of the cell. There are proteins involved in the making of proteins! In fact, all the molecular building blocks we have discussed in this course are manufactured by proteins, in most cases, through the actions of many proteins working together. Every biological function that is carried out in a cell—assembling molecules, shuttling them throughout the cell, or breaking them down—is carried out by proteins, and all the biological functions performed by proteins are dictated by their shape.
When Shape Goes Wrong
Typically, when a protein is folded into the wrong shape, it is simply broken down and recycled by the cell. However, in some cases, misfolded proteins can have deadly consequences. Several diseases are known to be caused by the accumulation of misfolded proteins that cannot be broken down. Mad cow disease, or BSE, is a well-known example of a disease caused by a misfolded protein that accumulates in the central nervous system of infected cattle. The protein is transmitted through the consumption of contaminated meat, whereupon it attacks the brain of the new host with devastating effect.
Over the last several lessons, we’ve discussed genes and their final products, proteins. However, we’re still missing a piece of the puzzle. In the next lesson, we’ll begin looking at the molecular building blocks responsible for assembling proteins according to their genetic instructions.
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